A variety of manufacturing environments require strict control over the presence of foreign debris in the air. Semiconductor manufacturing, for example, has long required “clean-rooms” that use extensive air filtering to reduce the number and size of particles in the air to some acceptable level. Other manufacturing environments have similar but distinct requirements. For example, in pharmaceutical or medical device manufacturing environments, hospitals, and food processing or preparation environments, it is critical to control not only the number of particles in the air, but minimization of biologic particles is of particular importance. Microbial contamination, for example, can render an entire batch of pharmaceutical product unusable leading to significant monetary losses in the manufacturing process. Additionally, it is advantageous to have instantaneous detection of contamination events, including instantaneous information about whether a contamination event is biologic or non-biologic, during the manufacturing process for pharmaceuticals or medical devices.
Various systems and methods exist to detect and determine the size of airborne particles. Systems are also available to detect and characterize detected airborne particles as biologic or inert. For example, systems have been designed to detect the presence airborne particles by measuring the amount and directionality of light scattered by particles to determine particle size and the measurement of fluorescence excited in particles by illumination with a source light to classify measured particles as biological or non-biological.
In most conventional systems, fluid to be sampled (e.g., environmental air), is pulled into the system and introduced into a sensing chamber, for example, through an inlet nozzle. The fluid is then measured for particles in some way, for example, by illuminating the fluid with a beam of light. The fluid is then evacuated from the sensing chamber through an outlet nozzle.
This arrangement poses certain challenges. For example, particles may escape the flow of fluid between the inlet nozzle and the outlet nozzle and enter the sensing chamber. In other cases, particles may back up against the flow of fluid, and enter the sensing chamber though the outlet nozzle. In both cases, such particles can cross the illumination beam and scatter light or fluoresce, just as they would if they were being measured in the interrogation zone between the inlet and outlet nozzle. These particles create spurious optical emissions, which may be detected by the optical detectors in the particle detection system, thereby degrading the accuracy of the measurement data.
The problem is illustrated in FIGS. 1(a) and 1(b), which show a particle detection system 100. System 100 includes a sealed sensing chamber 102. Fluid (e.g., a gas) containing particles 115 is drawn into sensing chamber 102 and introduced thereto through inlet nozzle 105. The fluid is then drawn out of sensing chamber 102 through outlet nozzle 110 after temporarily occupying an interrogation zone 112. Negative pressure, is supplied by non-illustrated suction pump or blower in fluid communication with outlet nozzle 110. Ideally, suction pump provides a first negative pressure in the vicinity of 114, which is lower than a second negative pressure in the interrogation zone 112, which is lower still than the negative pressure at an exit aperture of inlet nozzle 105, which is lower still than the pressure in the ambient environment from with the fluid is drawn. Ideally, this ensures orderly flow of fluid from the environment, through the interrogation zone 112, and out through outlet nozzle 105.
While in interrogation zone 112, the fluid is illuminated by a light beam, generated by a light source, for example, a laser, laser diode or LED, in the vicinity of 135. While illuminated, particles in the fluid scatter light in the direction of 140, where scattered light may be detected by a scatter detector. Additionally, while illuminated, biological particles in the fluid fluoresce, and this fluorescence light is collected by an ellipsoidal reflector 125 and directed to a fluorescence detector in the vicinity of 130.
In certain systems, stray particles 142 can escape the fluid flow in the interrogation zone, leave the vicinity of the interrogation zone, and occupy other areas of the sensing chamber 102. Stray particles 142 are particles that either never reach outlet nozzle 110 to be evacuated from the system, particles that back up from outlet nozzle 110 (as shown in FIG. 1(b)), or particles from some other source. Such particles can create spurious readings at the fluorescence and scatter detectors, thereby degrading the accuracy of the measurement data. The presence of particles within the sensing chamber 102 outside of interrogation zone 112 is particularly troublesome as such particles can become deposited on the interior walls that define sensing chamber 112, from which they are difficult to dislodge.
A conventional solution for dealing with the problem of particles escaping the flow of fluid into a sensing chamber utilizes a “sheath flow” of clean air, which encapsulates the flow of particles under test as they travel through an interrogation zone. In a conventional arrangement an inlet nozzle is provided having an inner or central portion embedded in and surrounded by an outer annular portion. The outer annular portion divided into an upper and lower section. Environmental air from the upper annular section is diverted from an input stream. This “sheath air” is then filtered and accelerated with a sheath pump. The sheath air is then re-introduced into a lower annular section of the inlet nozzle. Meanwhile, air containing particles to be measured proceeds in the inner portion. In the region of the interrogation zone (i.e., near the input aperture to the sensing chamber), the lower annular section of both the central nozzle (carrying the air to be measured), and the outer annular portion taper to accelerate both flows of air. The combined air flows are then introduced to the interrogation zone, sampled, and then evacuated using a total flow pump. The result is that the air containing particles to be sampled is encapsulated in a relatively more quickly flowing sheath of clean, filtered air. This prevents particles from escaping the interior flow of sample air before both flows are drawn from the sensing chamber. A device operating according to this method is described in U.S. Pat. No. 5,561,515 to Hairston, et al., at FIG. 6.